Liquid silicone elastomers, commonly referred to as liquid silicon rubbers (LSR) are mixtures essentially made up of a polyorganosiloxane having at least two unsaturated bonds, a hydrogenopolyorganosiloxane having at least two silicon-hydrogen bonds, and a hydrosilylation catalyst. It should be observed that compositions of this type also generally contain silica in order to give a certain amount of cohesion to the final material, it being understood that cross-linked silicones intrinsically present a consistency that is somewhat pasty or even oily in the normal, non-cross-linked state.
In any event, silicone elastomers are known for providing excellent resistance to high temperatures and to bad weather, together with good insulating properties. However that type of material also presents the drawback of presenting only poor tear strength, in particular in comparison with natural and synthetic rubbers which are themselves mostly constituted by a carbon backbone. In practice, this tear characteristic raises a genuine problem when silicone elastomers are for use in making embodiments that are likely to be handled. This applies specifically to applications of the electric power accessory type.
Nevertheless, various technical solutions exist for increasing the tear strength of materials based on liquid silicone elastomers.
The first consists in using large quantities of silica filler. However because of the thickening properties of compounds of that type, there often occurs an undesirable increase in the viscosity of the composition, which can even lead to the material hardening. The liquid silicone elastomer can thus become incompatible with a low-pressure injection method, which is a technique used for working this type of material. Increasing the silica content within the liquid silicone elastomer composition also has the consequence of degrading some of the electrical properties of the cross-linked material, in particular its resistance to arcing and its resistance to creepage or trapping.
The tear strength of silicone elastomers can also be increased by using long polysiloxane chains, since they are capable of providing greater functionality specifically because of their greater dimensions and thus a greater density of cross-linking. However, as with the first solution, this leads quickly to a significant increase in viscosity, or even to an increase in the hardness of the cross-linked material.
A third known solution enables all of those recurrent drawbacks to be avoided. It consists in adding a polymer to the liquid silicone elastomer composition, such as a polyamide, a polyethylene, or an EPDM, in association with one or more coupling agents. Although that type of solution does indeed enable the tear strength of the material to be increased, it nevertheless presents the drawback of being comparatively much more expensive than its equivalents in the state of the art. This is both because of the intrinsic cost price of each coupling agent used, and also because of the cost associated with the use of such additives making the fabrication process more complex.